Analytical Approach To The Design Of Atension Leg Platform

Abstract

INTRODUCTION The economics of offshore petroleum production have changed radically over the past few years, and reservoirs thought too deep or small now contain the financial incentive to justify their development. The various techniques and concepts previously developed to exploit such conditions pave also become economically feasible, such as the Tension Leg Platform (TLP). The costs of TLP systems can be justified for use in the deep water reservoirs which are currently attracting attention because, at the water depths being considered, TLP's are less expensive than alternative structures. The results of a comparative study done by Daily, Karsan and Kypkel, illustrated on Fig. I, compares the qualitative cost between conventional fixed structures, Guyed Towers and Tension Leg Platforms. The cost comparison crossover points for the offshore structure alternatives shown on Fig. 1 depend on environmental factors as well as water depth. The severity of the sea environment, seabed conditions and topography possible seismic activity, projected life of the field and desireability of platform reuse are all cost factors. In general, the cost crossover points move to the left, i.e. to shallower water depths, with an increase in degree of any of the environmental variables other than water depth. It is in the deeper water depths that the TLP cost curve increases at a lesser rate than conventional structures, because the platform structure does not change as you move into deeper and deeper water. In other words, fixed platforms and guyed tower structure size correlate with water depth; the deeper the water, the larger the structure size required. In fact the inability to attain the structure size and proportions required by conventional platform design probably set the upper limits on the water depths in which conventional systems can operate. Thus the cost curves for these two structures will increase more- rapidly than the TLP in deep water reservoirs, for once a TLP platform is paid for, nothing will change except the amount of the mooring system to be bought and the cost to install it. The mooring system is the common concept linking all TLP design. TLP's are basically buoyant structures restrained at a draft greater than that required for hydrostatic equilibrium by use of a vertically oriented, taut mooring system. This mooring system is quite rigid axially while being relatively flexible transversely. TLP design studies by Peres and Kypke2 and Capanoglu3 show that the mooring system configuration effectively eliminates heave motion, and the horizontal offset can be limited to an acceptable percentage of water depth. The TLP has been developed as variations to one or more of the basic configurations noted in Fig. 2, and the individual components of a TLP drilling and production system are noted schematically in Fig. 3. These components will be found in a variety of shapes, locations, and applications on anyone of the TLP variations developed to date.